The present invention relates to sensing of top-of-geopressure surfaces in exploration for petroleum or natural gas, and more particularly, to locating such top-of-geopressure surfaces using reflection seismic data gathered from a region of exploration and to predicting locations of oil and natural gas fields based on the topography of such top-of-geopressure surfaces.
Most major hydrocarbon producing basins of the world have deep stratigraphic structures that are over-pressured. That is, the pore pressures within deeply buried rocks are greater than the hydrostatic pressure at comparable depths, the hydrostatic pressure at a given depth being equal to the density of the water times the depth times the gravitational constant. In order for such over-pressured formations to exist, the pore spaces within the formations must be hydraulically isolated from the interconnected network of pore spaces that extend from the surface to moderate depths in all basins. These formations are called "geopressured" because the excess pore pressures often approach the rock overburden pressure itself (i.e., the density of the rock times its depth times the gravitational constant).
It is often thought that seals of either chemical precipitates or very low permeability rock form at the top of such geopressured chambers, since any interconnectedness (i.e., permeability) would allow the excess pressures to equilibrate to hydrostatic pressures.
Processes which may cause geopressuring are: 1) excessive sedimentation burying rocks at such great depths and at such a fast rate that the pore pressures are not capable of equilibrating with the hydrostatic pressure gradient; 2) generation of excess pore pressures from chemical reactions that produce large positive volume changes, such as the maturation of hydrocarbons, and particularly, the production of natural gas from kerogens causing very large positive volume changes deep within a basin; and 3) thermal expansion of pore fluids caused by proximity to a strong heat source, such as salt structures. All three of the foregoing processes may contribute in varying degrees to the generation and maintenance of geopressured formations in basin interiors.
It has been recognized for some time that major hydrocarbon concentrations in geopressured basins are found in the high permeability traps above the top-of-geopressure surface in such basins. See, for example, J. M. Hunt, Am. Assoc. Petr. Geol. Bull., Vol. 74, pp. 1-12, 1990. Furthermore, the top-of-geopressure surfaces of such basins have considerable topography. For example, in the southern gulf coast of offshore Louisiana, relief of from 4,000 to greater than 10,000 feet has been found from well logs that penetrate this geopressured surface.
A known technique for locating the top-of-geopressure surface in a region of geological survey is to drill a well into the over-pressured sediment to detect excess pressures. The weight of drilling mud required to prevent blowouts is often used to determine the top-of-geopressure surface.
A more accurate known technique for determining the top-of-geopressure surface is to observe the decrease in porosity of sediments from velocity, density or resistivity well logs. See, for example, T. K. Kan et al., Am. Assoc. Petr. Geol. Abstr., Annual Meeting, 1990. The normal loading from sediments in a basin produces an exponential decrease in porosity with depth, and no geopressures. In a geopressured chamber, the porosity is anomalously high since more fluid is present in such a chamber than would be present in the absence of geopressure. Such anomalous porosities can be detected by well logs.
There have been several attempts to predict the top-of-geopressured surface from seismic reflection profiles. Velocity or density anomalies associated with the transition from normal pressures to over-pressures sometimes produces a sharp impedance boundary, which in turn causes a strong reflection event. See, for example, the aforementioned Kan et al. reference. A synthetic seismogram is generated from well logs in the vicinity of the seismic reflection profile to predict the amplitude and phase of the reflector corresponding to the top-of-geopressure surface. Furthermore, compressional to shear velocity ratios (i.e., Poisson's ratio) can be produced from the use of both traditional shear seismic sources on land, or from the prediction of pseudo-shear velocity logs based on logs of other physical properties gathered in the offshore. Since Poisson's ratio is known to change across the top-of-geopressure surface, such surface may be located by analyzing seismic reflection profiles to determine the ratio of compressional velocity to shear velocity.
It is known that complex trace analysis may be performed on reflection seismic traces to derive "attributes" of the normal seismic trace, such as reflection strength, instantaneous phase, instantaneous frequency, etc. See, for example, M. T. Taner et al., "Complex Seismic Trace Analysis," Geophysics, Vol. 44, No. 6, pp. 1041-1063 (June, 1979). A change in the reflection strength or instantaneous phase attribute is sometimes observed across the top-of-geopressured surface. Complex trace analysis is carried out by first performing a Hilbert transformation on the seismic time series from each received waveform train, in either relative amplitude profile (RAP) or true amplitude profile (TAP) format. The Hilbert transformation of the seismic time series is the spectral analysis over a moving time window of both the real and imaginary components of a complex time series, which produces the instantaneous amplitude and phase of the complex time series as a function of frequency within each window. As the time window is stepped along the seismic time series, the variation in instantaneous amplitude or phase may each be represented by respective time series. Typically, the window is 0.25 seconds long and is stepped along a seismogram at 0.1 second increments so that nine-tenth of the new window overlap with the signal from the previous window. The magnitude of the complex seismic time series obtained through complex trace analysis is referred to as the reflection strength attribute. The reflection strength time series has heretofore been analyzed to detect the instantaneous changes in amplitude which would be expected across major impedance boundaries produce by natural gas buildup zones in the subsurface and which would show up in sectional profile displays of the reflection strength as "bright spots."
The above-described techniques of locating top-of-geopressure surfaces by well logging have the drawback in that wells are expensive to drill, particularly in offshore regions. Consequently, in a given region of geological survey relatively few wells are drilled, and the top-of-geopressure surface for the region must be obtained by interpolating geopressure values between wells which are in general widely spaced. Moreover, wells are usually drilled at significant distances away from faults. Drilling of wells close to faults is generally avoided, and therefore well logging data in fault zones or close to fault zones are not usually available.
Known techniques for using complex trace analysis on reflection seismic traces and examining the attributes of such traces to extract geopressure information have the shortcomings of not providing reliable mapping of the top-of-geopressure surface. Although the change in impedance across the top-of-geopressured surface sometimes produces reflection strength or phase anomalies that can be recognized, the wavelength of seismic energy, which is typically approximately 25 meters, is too short to produce coherent reflection events in either real or attribute space in regions where the transition to over-pressured sediments occur over a depth interval which is much longer. For example, the transition zone in the United States gulf coast is between 100 and 1,000 feet thick.
Accordingly, a need clearly exists for a reliable technique of sensing the location of the top-of-geopressure from the surface, both on land and offshore. Furthermore, a need also exists for a more reliable technique for locating deposits of natural gas and petroleum based on the top-of-geopressure surface.